1. Field of Invention
The invention relates to a scanning device, and more particularly to a scanning device having a document pressing unit, wherein the pressing unit is configured to be disposed at a level lower than that of a scan platen of the scanning device in order to press a document.
2. Related Art
In a conventional automatic document feeder, rollers are usually utilized to transport a document across a scanning window so that a scanning module disposed below the scanning window scans the document to acquire an image of the document. The scan quality of the image of the document is associated with the depth of field of the scanning module. The greater depth of field allows the image sensor of the scanning module to acquire a sharp image of the document when the document is floating from the scanning window, whereas the smaller depth of field requires the document to be transported stably and in close contact with the scanning window.
When the document is transported across the scanning window, a conventional scanner is provided with a roller, which contacts with the scanning window, in order to effectively control the document to be positioned within the range of the depth of field of the scanning module. When the document is not transported across the scanning window, the roller is in sliding contact with the scanning window. When the document is transported across the scanning window, the roller is in rolling contact with the document, and the document is in sliding contact with the scanning window. In this case, the wear of the roller and the wear of the scanning window are very large, and may influence the scan quality. In addition, when the document contains thick sheets of paper, the document may not easily enter a nip between the roller and the scanning window so that the transporting procedure cannot be completed. Therefore, the document being transported is usually controlled to be positioned within the depth of field of the scanning module by elastically pushing the document against the scanning window in the prior art using either a flat plate or the roller. The contact surface area between the flat plate and the document is too large, and the document being transported across the scan region cannot be ensured to completely slide on the scanning window. More specifically, when a contact image sensor (CIS) module is utilized, the scan quality tends to be influenced more easily, and the conventional flat plate or roller cannot satisfy the requirement for obtaining quality images.
Alternatively, some scanning windows are hollow windows without glass. In this case, the roller rubs against the scanning module. Therefore, the prior art method accelerates the wear between the roller and the scanning window or between the roller and the scanning module, and thus influences the scan quality.